1. Field of the Invention
The invention relates to a novel method for recovering and recirculating a deuterium-containing gas. The invention also relates to a system for recovering and recirculating a deuterium-containing gas. The invention finds particular applicability in an annealing process during semiconductor device manufacturing.
2. Description of the Related Art
Basic processing steps involved in the manufacture of integrated circuits (ICs), for example, metal oxide semiconductor (MOS) ICs, include epitaxial growth, wafer cleaning, photolithography, ion implantation, diffusion, chemical vapor deposition (CVD), sputtering, etching and electrical testing.
Following metallization and prior to electrical testing of the devices, annealing of the wafers containing the devices under a "forming gas" atmosphere is known. A typical forming gas atmosphere includes from 5 to 10 percent by volume hydrogen (H.sub.2) diluted in nitrogen (N.sub.2).
The purpose of this annealing step is to passivate defects, or surface states, at the MOS interface generated during earlier IC manufacturing steps. Such surface states include, for example, dangling silicon bonds at the silicon surface and interface trap charge Q.sub.it present at the Si/SiO.sub.2 interface. Such defects can degrade the operating characteristics of the formed device by effectively removing charge carriers needed for the operation of the device or, alternatively, by the generation of unwanted charge carriers.
Notwithstanding the use of such an annealing process, high energy charge carriers may induce a "depassivation" of the previously passivated defects during normal operation of, for example, a MOS transistor. This depassivation can lead to subsequent, progressive degradation of the electrical properties of the transistor. The aging characteristics of the IC, in turn, become detrimentally affected. See, e.g., R.A.B. Devine et al, Applied Physics Letters, 70(22), p. 2999 (1997).
The above-described phenomena have become more and more problematic with the increases in electric field intensity brought on by increased device integration and the shrinking geometries associated therewith. Device lifetime may thus become unacceptably short for future generations of integrated circuits.
To address these problems, it has recently been proposed to conduct the post-metallization annealing step using pure deuterium (D.sub.2) or a deuterium-containing gas mixture in place of pure hydrogen (H.sub.2) or a hydrogen-containing gas mixture. In so doing, it has been shown that submicron MOS transistor lifetime can be significantly increased over that attained using hydrogen. The use of a deuterium-containing annealing atmosphere is discussed, for example, in International Publication No. WO 94/19829, which discloses contacting a silicon wafer with a deuterium-containing material to form Si--D and Si--OD bonds on a silicon surface at an interface with a silicon dioxide layer.
While the use of deuterium in the post-metallization annealing process is of great interest, its application on an industrial scale has, to date, been impractical. This impracticality is a result of the high cost and limited availability of the deuterium isotope. In particular, for the deuterium flowrates required, the cost of treating chamber exhaust as waste gas can be prohibitive.